1. Field of the Invention
The present invention relates to a method for fabricating a FinFET transistor device, and more particularly to a method of fabricating a double gate MOSFET with a capability of inhibiting the depletion effect of the conductive gate while operating this device, which can hence elevate the device drive-on currents.
2. Description of the Prior Art
In the past several years, significant progress has been made for the scaling of classical planar MOSFET (metal-oxide-semiconductor field effect transistor) structure to the gate lengths below 65 nm. Despite difficulties in fabrication, sub-20 nm physical gate length MOSFETs have recently been demonstrated. However, further scaling of the planar structure below 50 nm becomes increasingly challenging due to excess leakage, degradation in mobility, and a variety of difficulties within the device processing. Therefore, alternative MOSFET structures and new process technologies need to be explored. The effective control of leakage in nano-scale transistors will be extremely important for high-performance densely packed chips such as microprocessors.
The double gate MOSFET, featuring excellent short-channel behavior and relaxed requirements for aggressive scaling of gate dielectrics and junction depths, is attractive for high-performance low power applications. Fully-depleted vertical double gate devices with symmetric gate structures feature a low vertical electrical field in the channel which is favorable to carrier transport. The FinFET is a promising vertical double gate structure, which has been demonstrated in the last few years at gate lengths ranging from 100 nm to 10 nm. Unlike other reported vertical double gate structures, the FinFET can be fabricated with minimal deviation from the industry standard CMOS process.
The FinFET transistors were fabricated on SOI (silicon-on-insulator) wafers with a modified planar CMOS process. FIG. 1A to FIG. 1F is an illustration of the general process flow for fabricating the FinFET transistor. At first, referring to FIG. 1A, a substrate such as a silicon on insulator (SOI) structure is provided, including a silicon substrate 10, a buried oxide layer (BOX) 11 and a silicon layer 12 on the buried oxide layer 11. A cap oxide layer 13 was thermally grown on the silicon layer 12 to relieve the stress between the ensuing nitride hard mask and the silicon layer 12. A silicon nitride layer 14 was deposited on the cap oxide layer 13, to serve as a hard mask.
After the hard mask deposition, a photoresistor 15 is applied to define the hard mask through use of optical lithography, electron beam lithography, X-ray lithography, or other conventional means to produce a chemical etchant mask. Then, referring to FIG. 1B, after the mask definition, an etch process is used to pattern the hard mask and the device fin structure including a silicon drain region (not shown) and a silicon source region (not shown) connected by a silicon fin or channel 12, and thereafter the photoresistor 15 is removed. Referring to FIG. 1C, a thin sacrificial oxidation process maybe used to form a sacrificial oxide 16 on the two parallelly opposing sidewalls of the silicon fin 12 to repair any damage done to the fin surface during the etch process. Oxidation may also be used to reduce the fin width, thereby allowing sub-lithography dimensions to be achieved. The threshold (Vt) implants of NMOS and PMOS can be subsequently proceeded. Referring to FIG. 1D, the hard mask of the silicon nitride layer 14 and the thin sacrificial oxide 16 are removed to retain the cap oxide layer 13 on the silicon fin 12. Referring to FIG. 1E, a gate oxide 17 is grown or deposited on the two opposing sidewalls of the silicon fin 12. Then, referring to FIG. 1F, a polysilicon gate material is deposited over the surface of the silicon fin 12, a gate mask is defined on the polysilicon gate material and then the underlying gate material is etched to form a polysilicon gate 18 with the etching stopping on the cap oxide 13 and the buried oxide layer 11. An ion implantation process is performed to implant dopants in the polysilicon gate 18 for a desired threshold voltage Vth. Referring to FIG. 2, the source/drain regions 12A and 12B are also doped to make them electrically conductive electrodes in the subsequent source/drain ion implantations. Referring to FIG. 1F again, however, the distribution of the dopants in the polysilicon gate 18 is a gaussian distribution along the depth of the polysilicon gate 18. The .portions of the polysilicon gate 18 nearby the bottom corners contained between the gate oxide 17 and the buried oxide layer 11 would have been more lightly doped, and have a larger resistance. Furthermore, upon operating the FinFET transistor, a depletion of the polysilicon gate 18 easily occurs, which in turn thickens the equivalent oxide thickness (EOT) of the gate dielectric 17, resulting in the reduction of the drive current of the FinFET transistor.
Accordingly, it is an intention to provide a method of fabricating a FinFET transistor device, which can alleviate the problem encountered in the conventional process for fabricating the FinFET transistor.